All combustion systems, including stationary gas turbine combustors, can operate in a mode where high pressure oscillations exist in the vicinity of and are sustained by the flame. These oscillations are driven either by a periodic fluctuation in the fuel or air flow caused by an external source or by a coupling of the heat release rate and an acoustical mode of the combustion chamber. In either case, the resulting pressure oscillations generate mechanical stresses in the combustion hardware and can also generate very high levels of noise. The magnitude of the stresses in the hardware varies considerably depending upon the degree of coupling between the acoustical mode and the heat release rate, and failures can occur in a time period as brief as a few minutes. Further, the weak coupling significantly limits the life of the apparatus parts as compared to their design values and therefore results in added expense for inspections and repair or replacement.
Much of the effort devoted to reducing dynamic pressure oscillations in combustion systems have been directed toward the highly destructive pure tone resonances found in all types of combustors. There is, however, a much lower level narrow band pressure oscillation, caused by the same factors leading to the pure tone resonance, that significantly limits combustion hardware operating life.
Driven oscillations, i.e. those caused by external sources such as the fuel supply or the air supply, can generally be controlled by careful attention to design of the combustion system. The control of acoustical oscillations, however, can be more difficult particularly when the fundamental frequency of the combustor is less than 300-500 hz. In these cases, a weak coupling between the acoustic mode and the heat release rate usually occurs although there will be some operating conditions where a strong coupling (pure tone combustion resonance) exits.
Prior efforts for controlling dynamic pressures in combustion systems have been mainly concerned with rocket engines where the general approach has been to utilize known design methods to securely anchor the flame front downstream of a flameholder or to otherwise change the local fuel-air ratio in the flame zone and thus destroy the phase relationship between the pressure and heat release pulsations. Most of such methods are ineffective in those cases where there is only a weak coupling between the pressure and heat release.
Accordingly, it is the object of this invention to provide a combustor in which the narrow band dynamic pressure oscillations are reduced thereby extending equipment life and reducing noise. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description in which FIGS. 1-4 are schematic cross-sections of four different embodiments of the invention.